1. Field of the Invention
The present invention relates to a fixing mechanism, which fixes a wire guide on a wire guide supporting face of a wire guide supporting part, of a wire guide of a wire electric discharge machine.
2. Description of the Related Art
In a wire electric discharge machine which stretches a wire electrode in the Z direction and relatively moves the wire electrode and a workpiece so as to machine the workpiece, the wire electrode is restrained by a part which is called a wire guide so as not to move in the XY direction. The wire guide is detachably fixed on a wire guide supporting part by a fixing means such as a screw. The wire guide is commonly formed such that a circular hole which is larger than a diameter of a wire electrode line by approximately several μm to 10 μm is formed in a high hardness material such as ruby. When the wire electrode is changed to a wire electrode having a different diameter, the wire guide is exchanged into a wire guide adapted to the diameter of the new wire electrode. The diameter of a wire guide is selected and exchanged depending on various conditions such as a plate thickness, a shape, a machining speed, and machining accuracy of a workpiece which is to be machined, and running cost. Further, the wire guide is abraded through a long period of use, so that the wire guide is exchanged for a new wire guide and the old wire guide is discarded.
A position of the wire electrode in the XY direction is determined depending on a position of a wire guide which restrains the wire electrode. Therefore, when a new wire guide is used, it is necessary to measure a vertical position of a wire electrode and a position of a supporting point of taper machining according to the wire guide. In a case where highly-accurate machining is performed, it is necessary to highly accurately measure a vertical position and a supporting point position as well. The measurement takes at least 30 minutes, commonly, one hour. After the measurement of a vertical position and a supporting point position is completed, the measurement values are inputted and stored in a control device of a machine and are used in actual machining of a workpiece. Unless a position of a wire guide is changed, the values of the vertical position and the supporting point position can be continually used as they are even when a workpiece and a machining content are changed.
In order to reduce a trouble of exchanging wire electrodes in machining of a workpiece, Japanese Patent Application Laid-Open No. 58-196926 discloses a wire electric discharge machine which is provided with a plurality of pairs of wire guides so as to automatically supply wire electrodes having different wire diameters from each other and thus can change the wire electrode diameter without getting any assistance from someone in the middle of machining.
Here, an example of a wire guide and a wire guide supporting part on a lower side according to the prior art configuration is described with reference to FIGS. 1 to 3.
A wire guide 8 and a wire guide supporting part 6 respectively have abutting faces on the XY plane. The wire guide 8 is provided with a male screw and the wire guide supporting part 6 is provided with a female screw. The wire guide 8 is screwed until the abutting face 8a of the wire guide 8 and the abutting face 6a of the wire guide supporting part 6 abut on each other, so as to be fixed on the wire guide supporting part 6. In a case of this configuration, high restoration accuracy of a position in the Z direction which is orthogonal to the XY plane of the abutting faces can be expected, but an error caused by a gap between the male screw and the female screw is generated in the XY direction. In addition, there is a gap between a center axis of a hole which supports a wire electrode 4 and a center axis of a screw hole. Therefore, a phase on which the wire guide 8 is fixed is shifted due to variation of a fastening torque for fixing the wire guide 8 on the wire guide supporting part 6, generating an error in the XY direction
A wire guide and a wire guide supporting part according to prior art commonly have such configuration that a screw part for fixing the wire guide has both of roles of a positioning mechanism for positioning the wire guide in the XY direction and a fixing mechanism for fixing the wire guide. It is difficult to manufacture a part of which fitting accuracy between a male screw and a female screw is enhanced, requiring higher cost. Therefore, a screw part of a wire guide and a screw part of a wire guide supporting part of prior art have a common loose-fitting structure. Consequently, a wire guide is not always restored on the same position when the wire guide is detached and attached again, commonly generating an error of 10 μm or more between before and after detachment of the wire guide from the wire guide supporting part. Therefore, when a wire guide is exchanged for another wire guide, a previous measurement value which is stored in a control device cannot be used as it is and it is necessary to perform measurement requiring long time in every exchange.
Thus, even though wire guides need to be frequently exchanged depending on a machining content, a fixing mechanism of a wire guide according to prior art has the configuration that requires considerable steps in every exchange of wire guides, deteriorating productivity of a wire electric discharge machine.